1. Field of the Invention
The prevent invention relates to an imaging device, and more particularly, it relates to an imaging device comprising a multiplier section for multiplying electrons.
2. Description of the Background Art
A CCD (charge coupled device) image sensor (imaging device) comprising a multiplier section for multiplying electrons is known in general.
FIG. 12 is a sectional view showing a structure of the conventional CCD image sensor. With reference to FIG. 12, in an exemplary conventional CCD image sensor, a gate oxide 102 is formed on a surface of a silicon substrate 101. Four gate electrodes 103 to 106 are provided in prescribed regions on an upper surface of the gate oxide 102 at prescribed intervals. The CCD image sensor is so formed that four-phase clock signals Φ1 to Φ4 are supplied to the gate electrodes 103 to 106. A pixel separation barrier, a temporary storage well, a charge transfer barrier and a charge accumulation well are formed in a transfer channel 107 located under the gate electrodes 103 to 106, respectively. This pixel separation barrier separates the temporary storage well and the charge accumulation well of an adjacent pixel, and has a function of transferring electrons in the charge accumulation well of the adjacent pixel to the temporary storage well. The temporary storage well has a function of temporarily storing electrons transferred from an adjacent pixel. The charge transfer barrier separates the temporary storage well and the charge accumulation well, and has a function of transferring electrons stored in the temporary storage well to the charge accumulation well. The charge accumulation well has a function of storing electrons transferred from the temporary storage well and also has a function as a multiplication region for multiplying electrons due to impact ionization by an electric field. A multiplier section is constituted by the charge accumulation well and the gate electrode 106. In other words, a high electric field region 109 adjusted to a high potential is formed in an interface between the charge transfer barrier and the charge accumulation well, and when the electrons stored in the temporary storage well is injected into the high electric field region 109, the injected electrons obtain energy from the high electric field region 109. Then, the electrons having obtained the energy collide with atoms of the silicon substrate 101 during movement in the high electric field region 109, and electrons and holes are generated by this impact. Among the generated electrons and holes, only the electrons are accumulated in the charge accumulation well by the electric field in the high electric field region 109. Thus, the electrons are multiplied. The multiplication of the electrons is performed in a process of transferring electrons generated by a photodiode 108 of a photodetection region.
A multiplication operation of the conventional CCD image sensor will be now described with reference to FIG. 12.
A high-level clock signal Φ1 is supplied to the gate electrode 103 to bring the gate electrode 103 into an ON-state, and the gate electrode 106 of the adjacent pixel is brought into an OFF-state. Thus, the electrons stored in the charge accumulation well of the adjacent pixel are transferred to the pixel separation barrier.
A high-level clock signal Φ2 is supplied to the gate electrode 104 to bring the gate electrode 104 into an ON-state, and a low-level clock signal Φ1 is supplied to the gate electrode 103 to bring the gate electrode 103 to an OFF-state. Thus, the electrons transferred to the pixel separation barrier are transferred to the temporary storage well.
Then, a high-level clock signal Φ4 is supplied to the gate electrode 106 to bring the gate electrode 106 into an ON-state. Thus, a high voltage is applied to the gate electrode 106, and the high electric field region 109 is formed in the interface between the charge transfer barrier and the charge accumulation well. After that, while the gate electrode 106 remains in the ON-state, a low-level clock signal Φ2 is supplied to the gate electrode 104 to bring the gate electrode 104 into an OFF-state, whereby the electrons stored in the temporary storage well are transferred to the charge accumulation well over the charge transfer barrier. Thus, the transferred electrons are multiplied due to impact ionization by a high electric field, and the multiplied electrons are stored in the charge accumulation well. A constant voltage is supplied to the gate electrode 105, whereby the charge transfer barrier is adjusted to a prescribed potential and stays constant.
FIG. 13 is a sectional view in a case where a structure of the conventional CCD image sensor shown in FIG. 12 is applied to a CMOS (complementary metal oxide semiconductor) image sensor (imaging device). With reference to FIG. 13, in a CMOS image sensor to which the structure of the conventional CCD image sensor is applied, an n-type impurity region 201a is formed in a prescribed region in the vicinity of a surface of the silicon substrate 201 and a gate oxide 202 is formed in a region corresponding to the n-type impurity region 201a on the surface of the silicon substrate 201. A gate electrode 207 for transferring electrons to a floating diffusion region 208 and reading data is further provided in a prescribed region on an upper surface of the gate oxide 202 in addition to four gate electrodes 203 to 206 having a function similar to the conventional CCD image sensor. In the CMOS image sensor to which the structure of the conventional CCD image sensor is applied, the photodiode 209 generating electrons, the floating diffusion region 208 and the aforementioned five gate electrodes 203 to 207 are provided in one pixel.
In the conventional CCD image sensor shown in FIG. 12, however, the three gate electrodes 103 to 105 for forming the pixel separation barrier, the temporary storage well and the charge transfer barriers respectively are disadvantageously required in order to transfer the electrons (carriers) transferred from the photodiode 108 to the charge accumulation well which is the multiplication region for multiplying electrons. Thus, it is disadvantageously difficult to miniaturize the imaging device (CCD image sensor). Also in a case where the structure of the conventional image sensor is applied to the CMOS image sensor, the three gate electrodes 203 to 205 for forming the pixel separation barrier, the temporary storage well and the charge transfer barrier respectively are disadvantageously required in order to transfer the electrons (carriers) generated in the photodiode 209 to the charge accumulation well which is the multiplication region for multiplying electrons, similarly to a case of the aforementioned conventional CCD image sensor. Thus, also in a case where the structure of the conventional CCD image sensor is applied to a CMOS image sensor, it is disadvantageously difficult to miniaturize the imaging device (CMOS image sensor) similarly to the case of the aforementioned conventional CCD image sensor.